private void Viewport3D_ContextMenuOpening(object sender, ContextMenuEventArgs e)
        {
            Point         location = Mouse.GetPosition(viewport);
            HitTestResult hit      = VisualTreeHelper.HitTest(viewport, location);
            var           meshHit  = hit as RayMeshGeometry3DHitTestResult;
            var           sphere   = spheres.Where(s => s.Geometry.Equals(meshHit.ModelHit)).FirstOrDefault();

            if (sphere == null) // if Model3D object weren't touched
            {
                e.Handled = true;
            }
            else
            {
                SelectedSphere = sphere;
            }
        }